A review of the synthesis, characterization, and mechanism of bimetallic catalysts for electrocatalytic CO2 reduction

Abstract

The electrocatalytic CO2 reduction reaction (CO2RR) is an environmentally friendly way to convert CO2 into valuable chemicals. However, CO2 conversion is a complex process, which contains 2, 4, 6, 8, and 12 electron transfer processes. It is very important to develop efficient catalysts to precisely control the number of electron transfers for the chemicals required. Single-metal catalysts have some deficiencies, including slow reaction kinetics, low product selectivity and inadequate stability. In response to these challenges, bimetallic catalysts have received significant attention owing to their unique structure and improved performance. The introduction of secondary metals alters the catalyst’s electronic structure, and creates novel active sites, as well as optimizing their interaction with the intermediates. This review provides a comprehensive account of atomically distributed bimetals based on carbon materials and non-atomic distributed bimetals such as alloys and heterostructures, including their synthesis methods, characterization, and the outcomes of different catalysts. Catalytic mechanisms of different bimetallic catalysts are proposed and challenges encountered in the CO2RR are considered.

1. Introduction

Carbon dioxide emissions resulting from the overconsumption of fossil fuels have led to serious climate hazards, the overarching effects of which are often irreversible within 1,000 years. As a result, it is imperative to either reduce CO2 emissions or convert it to valuable chemicals. The electroreduction of CO2 is an effective and clean strategy for CO2 reduction. CO2 reduction involves a variety of reaction paths (Table 1). Commonly there are 2, 4, 6, 8 and 12 electron transfer processes. The resulting products, such as CO and syngas (CO+H2), can be used to produce hydrocarbons or oxygenates by the Fischer-Tropsch synthesis method, and other chemicals such as methanol, formaldehyde, methane, ethanol, and several others can also be obtained from CO2 reduction. The products of CO2 reduction are sensitive to the practical applications and tends to produce a variety of products, which is one of the significant challenges in this area. On the other hand, considerable thermodynamic and kinetic barriers caused by the stable C＝O bond exist in the process of CO2 reduction. Additionally, a competing hydrogen evolution reaction (HER) is also unavoidable. Therefore, it is of great significance to develop efficient catalysts to improve CO2 reduction reaction (CO2RR).

Single-atom catalysts (SACs) have attracted significant attention in the field of CO2RR due to their good catalytic performance. The unsaturated coordination configuration of atoms in SACs makes them more active in many reactions. In addition, SACs allow for the design of active sites with well-defined positions through the tunable coordination environment, resulting in excellent activity and selectivity for specific reactions. For instance, Yang et al. designed a catalyst containing high dispersions of Fe-N4 moieties with a hierarchical structure, which prevented the migration and aggregation of Fe3+ due to the strong binding between metal ions and nitrogen. The porous structure of the catalyst also facilitated fast ion transfer and allowed for sufficient exposure of active sites, ultimately improving the selectivity of the catalyst. The Faraday efficiency of CO (FECO) achieved by this catalyst was up to 89%. However, SAC has only one active site and faces challenges in breaking the linear proportional relationship between intermediates when more complex reactions are conducted.

Recent researches have shown that the synergistic effect between adjacent atoms in bimetallic catalysts can significantly improve catalytic performance compared to isolated atoms. First, the introduction of secondary metals can alter the electronic structure of catalyst surface, thereby optimizing the adsorption and desorption of intermediates, and ultimately improving catalytic efficiency. For example, Gao et al. prepared In-Ga metal-organic-frameworks with abundant In-Ga bimetallic sites (InGa MOFs). The atomic bridging between Ga and In atoms effectively optimized the electronic structure of In, weakening In-C hybridization and enhancing In-O hybridization. This modification reduced the adsorption of the key intermediate *COOH involved in the competitive CO pathway, while enhancing the adsorption of *OCHO (formate pathway), in the end, promoting CO2 reduction.